How much for for kernel updates?

When Google put out new OS they often come with a higher kernel number. How much work is it for engineers to make the device compatible with the new Android OS? Someone said that it's like starting the project from the beginning because it is a new kernel. But it can be that much work right? There are only small changes to the kernel and many smartphone makers have offered updates from Froyo to Gingerbread.

They have to re-do everything. The linux kernel does not have a stable binary interface, so the various blobs used have to be updated, or the hooks that go into the kernel that attach those blobs have to be updated. The Android code was not merged into the kernel tree by Linus because it broke too many things and caused too many issues.